Glaze for nonmetallic resistors



June 4, 1935. E. HEDIGER GLAZE FOR NONMETALLIC RESISTORS Filed Jan. 5, 1953 INVENTOR. ERNST HEDIGER ATTORNEY Patented June 4, 1935 UNITED STATES PATENT OFFICE GLAZE FOR NONMETALLIC RESISTORS- Ernst Hediger, Niagara Falls, N. Y., assignor to Globar Corporation, Niagara Falls, N. Y., a corporation of New York Application January 3, 1933, erial No. 649,968 4 Claims. (Cl. 201-76) This invention relates to coatings for resistors icon carbide, and are bonded by a process known which are subjected to oxidizing conditions durin the art as self-bonding or recrystallization. ing their operation, and particularly to methods Even with these resistors, however, the resistance of improving the electrical properties and usetemperature coeflicient becomes less positive and ful life of resistors composed principally of silmay even become negative after a long period 5 icon carbide. The invention has for one of its of service. I have found that the use of my coatobjects the solving of certain problems encouning materially delays the alteration of this chartered in the operation of rigid self-sustaining acteristic. heating elements at high temperatures, and is My invention is illustrated bythe accompanyl0 particularly applicable to resistors composed of ing drawing which shows a section taken in a 0 relatively coarse particles. Coatings made in acplane perpendicular to the longitudinal axis of cordance with my invention materially increase one of my improved resistors. the strength of the resistor and also minimize As indicated in the drawing, the body of the the change in electrical resistance and change resistor contains comparatively coarse granules 3 in resistance temperature characteristics which and is surrounded by a coating 2 which is relal5 often take place in the operation of hightemtively much more compact and which is described perature heating elements. in detail below.

The changes, apart from direct mechanical The following table illustrates the manner in breakage, which limit the useful life of resistors which a coating of the type composing my inm composed of silicon carbide or other carbides vention improves the properties of recrystallized are not in general obvious upon visual examinasilicon carbide resistor rods designed for use on tion of the cold resistor. There is, however, a dis- 110 volt circuits with a current of 10 amperes. tinct change after a period of use which is usually Thezresistors in question are 14 inches long and manifested by an increase in resistance or by a of an inch in diameter.

change in the temperature resistance character- 5 istic of the resistor. The changes in electrical Trans use Increasein properties take place even at relatively low tem- Test No, f'f f strength peratures where there is no apparent oxidation ee en -l qa gg of the resistor material. 1 An increase in resistance of the resistor rod Percent 30 permits less current to flow through the red at a I Uncoated"- 2000 22 given voltage, so that the resistor, after a certain Owed 14 period of use, fails to heat up to the temperature II Uncoated..- 2500 as which is originally attained and which is desired. W 12 This efiect finally becomes so pronounced that 5 I the resistor must be replaced. One of the effects These results Show a notable increase of my type of coating is to retard this change strength 1n the coated resistors as compared w1th Another typical change accompanying t agthe uncoated resistors. The results show also that ing f a resistor is the'deveiopment of a negative the coated resistors retain their electrical char- 40' resistance temperature coeflicient. This means actelistics better than uncoated resistors D to 40 that the resistance becomes less as the tem- 1300110135 of perature increases. The existence of this neg- The increase in mechanical Strength Secured ative relationship has long retarded the applicathrough the addition of a Surface coat to the t of t type of resistor t many heating resistor is particularly advantageous because it problems because unless the voltage is reduced Permits the use Of materially coarser anules in 5 as the rod becomes hot there is a tendency for the the body of resistor, a p e t at esu ts in i t t away d destroy itself through a reduction of the rate of oxidation of the resistor overheating. As described in my copending apat elevated p a e plication, Serial No. 575,665, filed Nov. 17, 1931, The use Of granules f lar e slzes has been Patent 1,906,853, May 2, 1933, I hav found it found in certain cases to greatly increase the life 50 possible to produce a resistor having a po iti of recrystallized silicon carbide resistors. It has, temperature coeificient of resistance by certain however, been found that such resistors are frespecific burning methods and by the use of cerquently mechanically so weak as to require very tain types of silicon carbide. Such resistors are careful handling to avoid breakage. The addisubstantially devoid of materials other than siltion of a coating of fine material as described 55 below increases the strength of the resistor as indicated above and at the same time does not reduce the other desirable characteristics of the resistor.

As an illustration of the application of my invention, I take a slip made by mixing per cent by weight of fine silicon carbide and 20 per cent of bentonite or a self-fiuxing relatively insulating clay with sufiicient water to keep the mixture in suspension and apply this slip to the molded resistors by dipping the resistor in the slip or by spraying or brushing the slip onto the surface of the resistor. With resistors which are cured at a relatively low temperature, it is possible in some instances to apply the glaze before the final curing of the resistor. Under ordinary circumstance, however, the coating should be applied after the resistor has been burned or cured, and it may then be baked onto the resistor in a furnace or oven operated at a temperature sufiicient to vitrify the binder used. With clay, this is from 2000 to 2500 F.

As another example of a suitable mixture, I may use a mixture of finely divided silicon carbide and 'bentonite together with finely ground sillimanite, clay or clay grog.

The silicon carbide used in the slip should be in a state of fine subdivision, preferably such as is obtained by mixing crushed silicon carbide with water and decanting the fine material after the mix has been allowed to settle a few seconds. Bentonite, as is well known, is a type of claylike body which absorbs a large amount of water, is somewhat fusible, and tends to produce a glazed surface when heated to 2500 F.

The glaze is particularly applicable to recrystallized silicon carbide resistors, and especially to resistors containing a substantial proportion of particles many times the size of the particles in the glaze itself. In the curing of a resistor by recrystallization, the strength of the article is believed to be due to the interlocking of the crystalline particles by a process of crystal growth. The silicon carbide is infusible, and with mixes containing no appreciable proportion of fine material, it is difficult to secure a degree of selfbond ing or crystal growth which will provide a high degree of strength. When my glazing material is applied, the strength of a coarse grit rod can be increased to the point where it is comparable with a recrystallized resistor in which the original grit mix contained a substantial proportion of fine grit material. When my glaze is applied to a coarse grit resistor, the pores between the coarser particles in the original surface of the resistor are filled by the fine particles constituting the coating. The coating does not, however, penetrate to a material depth into the re sistor.

While in the specific examples given, bentonite or fusible clay was used as a binder for the fine silicon carbide, other vitrifiable binders may be added or the impurities inherent in commercial fine silicon carbide such as silica, iron oxide, etc. may be utilized as a binder.

Although my glaze is very desirable for increasing the life and mechanical strength of silicon carbide resistors, it is also suitable for resistors composed of other carbides, and for the protection of resistors composed principally of carbon or graphite.

I claim:

1. A silicon carbide heating element having a body portion composed mainly of coarse grains of silicon carbide and having a surface coating of finely divided silicon carbide bonded with a heat hardened ceramic bond.

2. A silicon carbide heating element having a body portion substantially devoid of materials other than silicon carbide, the major portion of said silicon carbide being relatively coarse, and having a surface coating of more finely divided silicon carbide which is bonded with a vitrifiable bonding agent.

3. A silicon carbide heating element having a body portion composed mainly of coarse grains of silicon carbide and having a surface coating of finely divided silicon carbide bonded with a heat hardened ceramic bond, the silicon carbide particles .in the body portion of the resistor being many times the size of silicon carbide particles in the surface coating.

4. A recrystallized silicon carbide heating element, having a body portion composed substantially entirely of recrystallized silicon carbide, and having a surface coating thereover, said surface coating being composed principally of finely divided silicon carbide and firmly attached to the resistor with a vitrifiable bonding agent.

ERNST HEDIGER. 

